1. Field of the Invention
This invention relates to improvements in an idle speed control system for an internal combustion engine, and more specifically to such an idle speed control system which is high in response in air supply to engine cylinders.
2. Description of the Prior Art
A variety of idle speed control systems have been proposed and put into practical use. A typical one of them is constructed and arranged as follows: An auxiliary air passage is provided to supply an auxiliary air for idling, into an intake air passageway through which intake air flows to be inducted into the engine cylinders of an engine. The auxiliary air is arranged to bypass a throttle valve rotatably disposed in the intake air passageway and provided with an auxiliary air control valve to control the amount of air flowing through the auxiliary air passage, thereby controlling idle speed. Such idle speed control system is disposed, for example, in Japanese Utility Model Provisional Publication No. 1-179148.
The auxiliary air control valve is of the electromagnetically operated type wherein the opening degree is controlled in accordance with duty cycle (factor) ISCon (%) applied thereto. The duty cycle is represented by a time rate (%) of a pulse width relative to a predetermined control cycle in case that the opening degree of the auxiliary control valve is controlled by changing the pulse width of a pulse signal which operate the auxiliary air control valve to open and is supplied in the predetermined cycle. The duty cycle ISCon (%) is calculated according to the following equation: EQU ISCon=ISCtw+ISCcl
where ISCtw is a basic control value which is set depending upon an engine coolant temperature Tw and with reference to a map on ROM; and ISCcl is a feedback correction value which is obtained by a proportional-plus-integral (PI) control of the result of comparison between an actual idle speed and a target idle speed under an idle speed feedback control condition. Thus, in the conventional idle speed control system, the idle speed control is achieved by the proportional-plus-integral control upon the comparison between the actual idle speed and the target idle speed.
However, drawbacks have been encountered in such a conventional idle speed control system, as set forth below. It has been a recent trend that a collector section (at which manifold branch runners are gathered) of an intake manifold has a large volume. The large volume collector section unavoidably retains a relatively large volume of air and therefore causes a delayed response in air supply to engine cylinders thereby resulting in engine speed lowering and/or hunting.